An image forming apparatus is arranged to form an electrostatic latent image on the surface of a photoconductor and giving toner to the electrostatic latent image for development. In an image forming apparatus 300 as shown in FIG. 1, a photoconductor 310 is charged by a charging device 320 and then the charged surface of the photoconductor 310 is partly exposed by an exposure device 330 to form an electrostatic latent image.
A concrete image forming method is described below. Firstly, the surface of the photoconductor 310 is uniformly charged by the charging device 320. Secondly, the exposure device 330 applies light corresponding to an image to the uniformly charged surface of the photoconductor 310. Thus, a potential of the light exposed portion is changed. Herein, in the surface of the photoconductor 310, a light exposed portion corresponds to an image area to which toner will be given and a light unexposed portion corresponds to a background area to which toner will not be given. In this way, the electrostatic latent image including the image area and the background area is formed on the photoconductor 310. Successively, the charged toner is given to the electrostatic latent image on the surface of the photoconductor 310 by a developing roller 340, thereby forming a visible image. The toner on the surface of the photoconductor 310 is then transferred to a paper by a transfer unit 350. The transferred toner is fixed to the paper by a fuser unit 360 to prevent peeling of the toner. After the transferring, toner remaining on the surface of the photoconductor 310 is collected by a cleaner 370 to a toner collecting container 371.
The non-contact developing type development is conducted by applying a developing bias to attract the toner from the developing roller 340 onto the electrostatic latent image on the photoconductor 310. Accordingly, it is important for ensuring the quality of a printed material to cause a needed amount of toner to exactly or faithfully adhere to the electrostatic latent image in order to output clear and appropriately dark printed images. It is also necessary to prevent toner from adhering to the background area.
In the case where this developing bias has an alternating current component, toner moves back and forth between the photoconductor 310 and the developing roller 340. This motion is shown in FIG. 2. Specifically, the toner is caused to fly from the developing roller 340 to the photoconductor 310 by a developing voltage and to fly back from the photoconductor 310 to the developing roller 340 by a collecting voltage. By repeating this series of steps, the toner finally adheres to the electrostatic latent image on the photoconductor 310 for development.
An example of the developing bias to be applied is shown in FIG. 3. A developing voltage “Vmin” and a collecting voltage “Vmax” have opposite signs and are applied alternately. Accordingly, the charged toner receives forces forward and backward alternately. According to those voltages, the toner is caused to fly between the photoconductor 310 and the developing roller 340.
Meanwhile, charging characteristics of toner will change due to friction and others. At an initial stage, toner has good charging characteristics and little variation in charging amount. In the image forming apparatus, at the initial stage, such toner with good charging characteristics is stored in the developing device. This toner has an appropriate charge quantity to be attracted onto the developing roller 340 (hereinafter, referred to as “normally-charged toner”). However, when the toner deteriorates, some particles of the toner would only have a low charge quantity (hereinafter, referred to as “low-charged toner”). This low-charged toner may adhere to the background area of the electrostatic latent image to which the toner actually should not adhere. This results in image fogging during printing, leading to degradation in the quality of a printed material.
Therefore, a method of preventing the image fogging resulting from the low-charged toner has been proposed. Patent Literature 1 discloses a developing device and an image forming apparatus arranged to adjust an absolute value |Vmax−Vd| which is a difference between a peak voltage Vmax that forms an electric field in a direction that causes toner to fly toward a photoconductor and a potential Vd of the background area of the electrostatic latent image. This configuration adjusts the value |Vmax−Vd| according to a decrease in toner charge quantity to prevent the image fogging. It is to be noted that the peak voltage Vmax disclosed in Patent Literature 1 corresponds to the developing voltage Vmin in FIG. 3 and the peak voltage Vmin in Patent Literature 1 corresponds to the collecting voltage Vmax in FIG. 3.